There has been a lot of interest in the use of power lines as communication media to reduce the cost of reliable communications. This is generally referred to as power line communications (PLC). There have been standardization efforts for PLC, such as Powerline-Related Intelligent Metering Evolution (PRIME). In most of the PLC standards, communication must be supported on three phases because three-phase transmission is a power line characteristic in most countries. However, because of coupling between the phases, the signals received on each of phases are not completely independent. For example, a signal in phase two may act as interference to the signal in phase one. There is a need to remove this interference. Furthermore, each of power-line transmission phases may act as an independent path from the transmitter(s) to the receiver(s). These independent paths can be used to enhance the received signal quality.
FIG. 13 illustrates a typical electric power distribution system connecting substation 1301 to residences 1302a-n. Medium voltage (MV) power lines 1303 from substation 1301 carry voltage in the tens of kilovolts range. Transformer 1304 steps the MV power down to low voltage (LV) power on LV lines 1305 carrying voltage in the range of 100-240 VAC. Transformer 1304 is typically designed to operate at very low frequencies in the range of 50-60 Hz. Transformer 1304 does not allow high frequencies, such as signals greater than 100 KHz, to pass between LV lines 1305 and MV lines 1303. LV lines 1305 feed power to customers via meters 1306a-n, which are typically mounted on the outside of residences 1302a-n. A breaker panel, such as panel 1307 provides an interface between meter 1306n and electrical wires 1308 within residence 1302n. Electrical wires 1308 deliver power to outlets 1310, switches 1311 and other electric devices within residence 1302n. 
The power line topology illustrated in FIG. 13 can be used to deliver high-speed communications to residences 1302a-n. Power line communications modems 1312a-n may be coupled to LV power lines 1305 at meter 1306a-n. PLC modems 1312a-n are used to transmit and receive data signals over MV/LV lines 1303, 1305. Such data signals may be used to support communication systems, high speed Internet, telephony, video conferencing, video delivery and similar services. By transporting telecommunications and data signals over a power transmission network, there is no need to install new cabling to each subscriber 1302a-n. Thus, by using existing electricity distribution systems to carry data signals, significant cost savings are possible. One method for transmitting data over power lines uses a carrier signal having a frequency different from that of the power signal. The carrier signal is modulated by the data to be transmitted. Alternatively, PLC modem 1313 may be coupled to the MV/LV power lines via home electrical lines 1308 to transmit and receive the data signals.
PLC modems 1312a-n at residences 1302a-n use the MV/LV power grid to carry data signals to and from concentrator 1314 without requiring additional wiring. Concentrator 1314 may be coupled to either MV line 1303 or LV line 1305. Modems 1312a-n may support applications such as high-speed broadband internet links, narrowband control applications, and low bandwidth data collection applications. In a home environment, modems 1312a-n may enable home and building automation in heat and air conditioning, lighting and security. Outside the home, power line communication networks provide street lighting control and remote power meter data collection.
A problem with using a power line network as a communications medium is that the power lines are subject to noise and interference. Power line cables are susceptible, for example, to noise from broadcast radio signals and from electrical equipment coupled to the power lines. Noise propagates along the power lines and combines with communications signals, which may corrupt the communications signals. Another problem with using power line networks is caused by the structure of the cable. On MV and LV power lines, the inner section of the cable comprises a group of phase lines, each carrying one of the three supply phases. At radio frequencies, the capacitance between these separate lines causes the signals on one line to leak or couple onto the neighboring lines. The coupling process between phase lines may introduce a phase shift or other interference. Therefore, after propagating along the lines, the components of a communications signal on each line will no longer be in phase with each other, but will be of different phase and amplitude. Such coupling and interference cause problems with receiving equipment, which must attempt to decode the modified received signal and reconstruct the original signal.
U.S. Pat. No. 6,373,377 B1 to Sacca, et al. relates to a power supply with digital data coupling for power-line networking. The power supply of this document is located within a computer system and is coupled to a network interface card by a digital coupler. A circuitry block converts digital signals from the digital coupler to analog signals. An analog front end amplifies the analog signals for transmission to a power line. The power supply of U.S. Pat. No. 6,373,377 B1 does not scale or weight data signals before distributing the data signals to different phases of the power line.
U.S. Pat. No. 6,983,027 B2 to Seki, et al. relates to a receiver adapted to receive an OFDM signal. Signals are received at an antenna and converted to an intermediate frequency band signal, which is then converted into a digital signal. Information is detected within the digital signal. A signal-to-noise (SNR) ratio is generated from the output of a differential detection circuit. The SNR indicates a reception quality of the OFDM signal. The receiver of U.S. Pat. No. 6,983,027 B2 does not scale or weight data signals before distributing the data signals to different phases of the power line.
U.S. Pat. No. 7,027,483 B2 to Santhoff, et al. relates to a system for ultra-wideband communications through power lines. This document compares transmitting data across power lines using ultra-wideband signals to transmission using orthogonal frequency division multiplexing (OFDM). It is noted that allowing an OFDM system to adapt to channel characteristics is prohibitively complex and expensive. Instead, the document proposes using ultra-wideband signals with transmission power, pulse envelope shape and pulse recurrence frequency that are optimized for power-line transmission. The ultra-wideband system of U.S. Pat. No. 7,027,483 B2 does not scale or weight data signals before distributing the data signals to different phases of the power line.
U.S. Patent Publication No. 2009/0060060 A1 to Stadelmeier, et al. relates to transmitting an OFDM-modulated signal from a transmitter to a receiver over a power line network. A single signal is modulated for transmission in a multiple input multiple output (MIMO) mode. The system is capable of transmitting the signal only on two channels or two wires of a home electrical system. A single receiver receives the MIMO transmission over four channels comprising three wires and a common mode path. The transmission system of U.S. Patent Publication No. 2009/0060060 A1 does not scale or weight one or more data signals simultaneously before distributing the data signals to different phases of the power line. This document does not disclose identifying the phase used by the PLC modem.
U.S. Patent Publication No. 2010/0054314 A1 to Korobkov, et al. relates to initializing a modem for OFDM transmission. A length or duration of a guard interval or cyclic prefix in an OFDM symbol is newly selected at each modem start up. The modem receives a pre-defined linear frequency modulated (LFM) sweep-signal or a pseudo noise signal that covers all the frequencies on the channel. The transfer function for the channel is estimated from the received signal. An input filter is synthesized based on the estimated transfer function. A number of samples of the guard interval are selected based on an impulse response on the communication channel. The modem initialization procedure of U.S. Patent Publication No. 2010/0054314 A1 does not scale or weight one or more data signals simultaneously before distributing the data signals to different phases of the power line.